PostalForm Letters API

openapi: 3.2.0
info:
  title: PostalForm Projects Public Letters API
  version: '2026-05-06'
  description: Public PostalForm Projects API for customer SDKs. Includes document uploads, quotes, mail orders, credits, API keys, and signed customer webhooks.
servers:
- url: https://projects.postalform.com
tags:
- name: Letters
paths:
  /api/v1/letters/quotes:
    post:
      summary: Quote a mailpiece
      description: Quote a letter from document size, country codes, mail class, and proof-mail settings. Country codes default to US when omitted. PostalForm automatically selects an eligible fulfillment path; API clients choose mailpiece options, not the underlying production network.
      security:
      - bearerAuth: []
      requestBody:
        required: true
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/CreateQuoteRequest'
      responses:
        '200':
          description: Quote.
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Quote'
      operationId: createLetterQuote
      tags:
      - Letters
  /api/v1/letters:
    post:
      summary: Create a test or live mail order from a quote
      security:
      - bearerAuth: []
      parameters:
      - name: Idempotency-Key
        in: header
        required: true
        schema:
          type: string
      requestBody:
        required: true
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/CreateLetterRequest'
      responses:
        '200':
          description: Idempotent replay.
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Letter'
        '201':
          description: Created order.
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Letter'
      operationId: createLetter
      tags:
      - Letters
  /api/v1/letters/{order_id}:
    get:
      summary: Retrieve a mail order, timeline, tracking fields, and customer webhook events
      security:
      - bearerAuth: []
      parameters:
      - $ref: '#/components/parameters/OrderId'
      responses:
        '200':
          description: Order detail.
          content:
            application/json:
              schema:
                allOf:
                - $ref: '#/components/schemas/Letter'
                - type: object
                  properties:
                    timeline:
                      type: array
                      items:
                        $ref: '#/components/schemas/MailOrderEvent'
                    webhook_events:
                      type: array
                      items:
                        $ref: '#/components/schemas/WebhookEvent'
      operationId: getLetter
      tags:
      - Letters
  /api/v1/letters/{order_id}/document.pdf:
    get:
      summary: Preview or download the PDF for a letter order
      description: Streams the prepared PDF when available, otherwise the original uploaded PDF while preparation is pending. Documents follow the workspace document retention window.
      security:
      - bearerAuth: []
      parameters:
      - $ref: '#/components/parameters/OrderId'
      - name: version
        in: query
        required: false
        schema:
          type: string
          enum:
          - current
          - original
          - prepared
          default: current
        description: current returns the prepared PDF when available and otherwise the original upload.
      - name: disposition
        in: query
        required: false
        schema:
          type: string
          enum:
          - inline
          - attachment
          default: inline
        description: Use attachment to download instead of previewing inline.
      responses:
        '200':
          description: PDF bytes.
          content:
            application/pdf:
              schema:
                type: string
                format: binary
      operationId: getLetterDocument
      tags:
      - Letters
  /api/v1/letters/{order_id}/return-receipt.pdf:
    get:
      summary: Download a stored USPS electronic return receipt
      description: Returns the signed USPS proof-of-delivery PDF after it has been acquired for an order using automatic ERR delivery. The PDF contains USPS delivery details and the recipient signature image or approved hand-stamp supplied by USPS. The order response exposes availability and the retention deadline.
      security:
      - bearerAuth: []
      parameters:
      - $ref: '#/components/parameters/OrderId'
      responses:
        '200':
          description: USPS electronic return receipt PDF.
          content:
            application/pdf:
              schema:
                type: string
                format: binary
        '404':
          description: The receipt has not been acquired or is not available for this order.
        '410':
          description: The stored receipt has passed its retention deadline.
      operationId: getLetterReturnReceipt
      tags:
      - Letters
components:
  schemas:
    WebhookEvent:
      type: object
      properties:
        id:
          type: string
        workspaceId:
          type: string
        orderId:
          type: string
        eventType:
          $ref: '#/components/schemas/CustomerWebhookEventType'
        payload:
          $ref: '#/components/schemas/CustomerWebhookPayload'
        createdAt:
          type: string
          format: date-time
        deliveries:
          type: array
          items:
            $ref: '#/components/schemas/WebhookDeliveryAttempt'
    CustomerWebhookEventType:
      type: string
      description: Customer webhook event names emitted for fulfillment status changes. Replace `letter` with `postcard` for postcard mailpieces.
      enum:
      - postalform.letter.accepted
      - postalform.letter.in_transit
      - postalform.letter.delivered
      - postalform.letter.returned
      - postalform.letter.failed
      - postalform.letter.canceled
      - postalform.postcard.accepted
      - postalform.postcard.in_transit
      - postalform.postcard.delivered
      - postalform.postcard.returned
      - postalform.postcard.failed
      - postalform.postcard.canceled
      x-enumDescriptions:
        postalform.letter.accepted: Letter accepted for production or mailing after the order leaves PostalForm's preparation queue.
        postalform.letter.in_transit: Letter entered the mail stream. The payload may include or update `mailpiece.tracking_number` when tracking is available.
        postalform.letter.delivered: Letter reported delivered by the carrier or delivery network.
        postalform.letter.returned: Letter returned or otherwise marked undeliverable.
        postalform.letter.failed: Letter could not be produced or mailed. Inspect the order timeline and retry with a corrected request when appropriate.
        postalform.letter.canceled: Letter canceled before delivery completion.
        postalform.postcard.accepted: Postcard accepted for production or mailing after the order leaves PostalForm's preparation queue.
        postalform.postcard.in_transit: Postcard entered the mail stream. The payload may include or update `mailpiece.tracking_number` when tracking is available.
        postalform.postcard.delivered: Postcard reported delivered by the carrier or delivery network.
        postalform.postcard.returned: Postcard returned or otherwise marked undeliverable.
        postalform.postcard.failed: Postcard could not be produced or mailed. Inspect the order timeline and retry with a corrected request when appropriate.
        postalform.postcard.canceled: Postcard canceled before delivery completion.
    CustomerWebhookPayload:
      type: object
      description: JSON body POSTed to customer webhook endpoints.
      properties:
        id:
          type: string
          example: evt_123
        type:
          $ref: '#/components/schemas/CustomerWebhookEventType'
        data:
          type: object
          properties:
            object:
              $ref: '#/components/schemas/Letter'
        mailpiece:
          type: object
          properties:
            status:
              type: string
              nullable: true
            tracking_number:
              type: string
              nullable: true
            tracking_status:
              type: string
              nullable: true
    Quote:
      type: object
      properties:
        standalone_address_page:
          type: boolean
          description: Resolved standalone address page setting saved on this quote. Always false for postcards.
        quote_id:
          type: string
        mailpiece_type:
          type: string
          enum:
          - letter
          - postcard
        postcard_size:
          type:
          - string
          - 'null'
          enum:
          - 4x6
          - 6x9
          - 11x6
          - null
        price_cents:
          type: integer
        currency:
          type: string
          enum:
          - usd
        pricing_version:
          type: string
        certified_return_receipt:
          type: boolean
        return_receipt_format:
          type: string
          enum:
          - electronic
          - physical
        restricted_delivery:
          type: boolean
        err_delivery:
          type: string
          enum:
          - manual
          - email
        err_email:
          type:
          - string
          - 'null'
          format: email
        signature_required:
          type: boolean
        expires_at:
          type: string
          format: date-time
    MailOrderEvent:
      type: object
      description: Customer-facing order timeline event. Internal fulfillment identifiers are not exposed.
      properties:
        id:
          type: string
        event_type:
          type: string
        status_before:
          type: string
        status_after:
          type: string
        source:
          type: string
        created_at:
          type: string
          format: date-time
    CreateLetterRequest:
      type: object
      description: Create an order from a quote. Requires Idempotency-Key header. Sender is strongly recommended for all live mail and required for some destinations and mailpiece options.
      properties:
        quote_id:
          type: string
        recipient:
          $ref: '#/components/schemas/MailingAddress'
        sender:
          $ref: '#/components/schemas/MailingAddress'
        metadata:
          type: object
          description: Optional caller metadata stored on the order and surfaced in reads/webhook payloads.
          additionalProperties: true
      required:
      - quote_id
      - recipient
    MailOrderStatus:
      type: string
      enum:
      - queued
      - document_preparing
      - document_prepared
      - submitted
      - accepted
      - in_transit
      - delivered
      - returned
      - submission_pending
      - failed
      - canceled
      description: Order timeline status. `submission_pending` means the fulfillment submit job entered a physical-mail safety window where PostalForm cannot blindly retry without risking duplicate mail; it requires reconciliation if it remains the current order status.
    CreateQuoteRequest:
      type: object
      description: Letter quote request. API clients choose mailpiece options; PostalForm handles fulfillment automatically.
      properties:
        document_id:
          type: string
        page_count:
          type: integer
          minimum: 1
          description: Optional explicit PDF page count. Used for deterministic pricing when present.
        mail_class:
          type: string
          default: usps_first_class
          description: Standard/USPS First Class by default. Accepts standard/usps_first_class, priority/usps_priority, and express/usps_express. Priority/Express cannot be combined with certified or registered proof mail.
        color:
          type: boolean
          default: false
        double_sided:
          type: boolean
          default: true
        standalone_address_page:
          type: boolean
          description: Keep the address page on its own sheet with a blank reverse for double-sided letters, preserving the document page pairing. Omit to inherit the workspace setting (off by default); explicit true or false overrides it. Single-sided letters are unchanged. The resolved choice is fixed on the quote, and added pages or sheets are included in pricing and provider limits.
        certified:
          type: boolean
          default: false
          description: Proof-mail add-on for letters only. Eligible U.S. standard letters request USPS Certified Mail. Eligible Canada standard letters request Canada Post Registered Mail. Eligible Belgium, Switzerland, Spain, and France standard letters request the available registered-mail option for that destination.
        certified_return_receipt:
          type: boolean
          default: false
          description: Return receipt for eligible U.S. Certified Mail letters. Defaults to the electronic format when true.
        return_receipt_format:
          type: string
          enum:
          - electronic
          - physical
          default: electronic
          description: Electronic selects the USPS signed proof-of-delivery PDF. Physical selects the mailed PS Form 3811 green card. Requires certified_return_receipt=true.
        restricted_delivery:
          type: boolean
          default: false
          description: Requests USPS Restricted Delivery for addressee-only delivery. Requires certified_return_receipt=true and uses the provider-managed restricted-delivery service.
        err_delivery:
          type: string
          enum:
          - manual
          - email
          default: manual
          description: Manual leaves receipt retrieval to the customer through the USPS tracking link. Email makes PostalForm acquire, retain, and email the electronic receipt when available.
        err_email:
          type: string
          format: email
          maxLength: 256
          description: Optional destination when err_delivery=email. If omitted, PostalForm uses the Projects account email. Not valid for a physical PS Form 3811.
        signature_required:
          type: boolean
          default: false
          description: Signature confirmation for eligible U.S. Priority and Express letters.
        destination_country_code:
          type: string
          minLength: 2
          maxLength: 2
          default: US
          description: ISO 3166-1 alpha-2 destination country code used for quote validation and pricing. Defaults to US. First-party Projects destinations are US, CA, AT, BE, CH, DE, ES, FR, GB, IN, LU, and NL.
        origin_country_code:
          type: string
          minLength: 2
          maxLength: 2
          default: US
          description: ISO 3166-1 alpha-2 sender/origin country code used for quote validation and pricing. Defaults to US.
      required:
      - document_id
    Mode:
      type: string
      enum:
      - test
      - live
    MailingAddress:
      type: object
      description: Flexible mailing address object. Pass countryCode, country_code, country, address_country, or addressCountry for international destinations; country defaults to US when omitted.
      additionalProperties: true
      properties:
        name:
          type: string
        company:
          type: string
        line1:
          type: string
          description: Also accepts street1, address_line1, or addressLine1.
        line2:
          type: string
          description: Also accepts street2, address_line2, or addressLine2.
        city:
          type: string
          description: Also accepts address_city or addressCity.
        state:
          type: string
          description: Also accepts province, provinceOrState, address_state, or addressState.
        postal_code:
          type: string
          description: Also accepts postalCode, postalOrZip, zip, address_zip, or addressZip.
        countryCode:
          type: string
          minLength: 2
          maxLength: 2
          default: US
          description: ISO 3166-1 alpha-2 country code. Defaults to US when omitted.
    WebhookDeliveryAttempt:
      type: object
      properties:
        id:
          type: string
        webhookEventId:
          type: string
        endpointId:
          type: string
        attemptNumber:
          type: integer
        status:
          type: string
          enum:
          - succeeded
          - failed
        httpStatus:
          type: integer
        responseBodySnippet:
          type: string
        attemptedAt:
          type: string
          format: date-time
        nextRetryAt:
          type: string
          format: date-time
    Letter:
      type: object
      properties:
        id:
          type: string
        object:
          type: string
          enum:
          - letter
          - postcard
        mailpiece_type:
          type: string
          enum:
          - letter
          - postcard
        postcard_size:
          type:
          - string
          - 'null'
          enum:
          - 4x6
          - 6x9
          - 11x6
          - null
        status:
          $ref: '#/components/schemas/MailOrderStatus'
        mode:
          $ref: '#/components/schemas/Mode'
        price_cents:
          type: integer
        currency:
          type: string
        funds_status:
          type: string
        billing_rail:
          type: string
        tracking_number:
          type: string
          nullable: true
          description: Carrier tracking number when available.
        tracking_url:
          type:
          - string
          - 'null'
          format: uri
          description: Direct USPS tracking URL when a tracking number is available. Manual ERR customers can use it to request their receipt from USPS.
        tracking_status:
          type: string
          nullable: true
          description: Normalized tracking status when available.
        return_receipt_format:
          type: string
          enum:
          - electronic
          - physical
        restricted_delivery:
          type: boolean
        err_delivery:
          type: string
          enum:
          - manual
          - email
        err_email:
          type:
          - string
          - 'null'
          format: email
        err_status:
          type: string
          enum:
          - manual
          - pending
          - sent
          - failed
          - expired
        err_delivered_at:
          type:
          - string
          - 'null'
          format: date-time
        err_retention_until:
          type:
          - string
          - 'null'
          format: date-time
          description: Last instant at which an acquired receipt remains retrievable. PostalForm's default ERR retention is three years.
        return_receipt_available:
          type: boolean
          description: Whether a stored electronic receipt can currently be downloaded.
        return_receipt_url:
          type:
          - string
          - 'null'
          description: Relative authenticated download URL when return_receipt_available is true.
        metadata:
          type: object
          additionalProperties: true
        created_at:
          type: string
          format: date-time
        updated_at:
          type: string
          format: date-time
        idempotent_replay:
          type: boolean
  parameters:
    OrderId:
      name: order_id
      in: path
      required: true
      schema:
        type: string
  securitySchemes:
    bearerAuth:
      type: http
      scheme: bearer